The present invention relates to circuit breaker thermal trip units, and, more particular, to a method and arrangement for calibration of circuit breaker thermal trip units.
In circuit breakers thermal trip units the bimetal elements deform, upon reaching a predetermined temperature, into contact with the circuit breaker trip bar to interrupt circuit current. An early teaching of one such bimetal trip unit for residential type circuit breakers is found in U.S. Pat. No. 3,908,110 entitled Method of Calibrating an Electric Circuit Breaker, wherein lasers and the like are used to calibrate the bimetal during manufacture. U.S. Pat. No. 5,317,471 entitled Process and Device for Setting a Thermal Trip Device, describes the use of lasers for calibrating bimetal trip units within industrial rated circuit breakers in accordance with European Industry Standards.
Further, U.S. Pat. No. 4,616,198 entitled Contact Arrangement for a Current Limiting Circuit Breaker, describes the use of a first and second pair of circuit breaker contacts arranged in series to reduce the amount of current let-through upon the occurrence of an overcurrent condition. U.S. patent application Ser. No. 09/087,038, filed May 29, 1998, entitled Rotary Contact Assembly for High Ampere-Rated Circuit Breakers, describes the operation of the circuit breaker trip unit to release the circuit breaker operating mechanism and separate the circuit breaker contacts.
When rotary operating mechanisms are used to control the circuit breaker contacts, such as described in the aforementioned U.S. Pat. No. 4,616,198 it is imperative that the trip units within each pole of the multi-pole circuit breaker open simultaneously to insure current limiting function within each one of the separate poles, as well as to avoid the occurrence of so-called "single phasing" whereby one of the phases interrupts independently of the remaining phases causing increased current transport through the remaining phases. Certain U.S. Federal and state electric codes require individual calibration of the bimetal trip units within each separate pole to insure simultaneous circuit interruption within each of the poles upon occurrence of an overcurrent condition in any one of the individual poles.
With the use of automatic bimetal calibration devices, as described in the aforementioned U.S. Pat. No. 3,908,110, for example, care is taken to insure that the bimetal element, per se, is electrically-insulated from the screws or pins use in calibrating the position of the bimetal element from the operating mechanism trip bar. The imposition of an electrically-insulative sleeve or the like between the bimetal and calibration screw increases both the installation and calibration expense at the time of manufacture.